Essentially two methods are known for separating such a power unit from the rest of the missile, after it has performed its function.
The first, which may be qualified as active, employs mechanical or pyrotechnical elements, such as cutter cords, explosive bolts, extraction springs, etc. . ., controlled by a logic device. Such active separation systems are therefore complex. In addition, their reliability is not perfect.
The second of said methods is called natural, as separation is effected spontaneously under the action of the aerodynamic drag of said power unit.
In missiles provided to employ such a natural separation, the power unit is connected to the rest of the missile by a precise fit, constituting a sliding connection parallel to the axis of said power unit, but rendering the latter fast in rotation with said missile.
In this way, upon launching of the missile and during its flight under the action of said power unit, the force of propulsion of the latter, reinforced by the aerodynamic drag of the rest of the missile and by the forces of friction of the sliding connection, but reduced by the aerodynamic drag of said power unit, ensures connection of the power unit on said missile.
On the other hand, at the end of operation of said power unit, said force of propulsion decreases very considerably (thrust decay) until it is canceled, with the result that it becomes insufficient to maintain said power unit fast with the rest of the missile. The aerodynamic drag of said power unit may then overcome the forces of friction of the sliding connection and, against the action of the aerodynamic drag of the rest of the missile, said power unit separates naturally from the rest of the missile.
Such a natural method of separation therefore presents considerable advantages of simplicity, both in production and in carrying out. However, it also presents considerable drawbacks, associated with the fact that the forces that it brings into play are difficult to master.
In fact, the forces of friction in a sliding connection depend on the state of the contact surface, the possible lubrication and on the clearance, i.e. the manufacturing tolerances of the fit of the power unit on the rest of the missile. Moreover, a phenomenon of gumming may appear, when the sliding connection is immobilized for a prolonged period of time, for example munitions in store.
Furthermore, the sliding connection which ensures rigidity of the assembly of the missile and the power unit, may be subjected to a considerable restraining moment during separation. This moment, which may result from the aerodynamics of the missile or from longitudinal mechanical vibrations, considerably affects the level of the forces of friction. Finally, the immobilization of the sliding connection in rotation may contribute additional frictions due to torsional couples (likewise of aerodynamic or vibratory origin) in the fit.
As to the forces of propulsion of the power unit at thrust decay, they too are not well known, like the aerodynamic forces and, more particularly, their distribution between the missile and the power unit.
In addition, the aerodynamic drag of the power unit, the only force on which one counts for separation and which is desirably great in order to promote separation, must, of course, be as reduced as possible for reasons of economy.
A certain number of uncertainties therefore appear, relative to the level of the forces employed in natural separation. Such incertainties are translated by a very considerable dispersion of the instant of separation. Such a separation may, in certain cases, not even occur at all. In any case, the subsequent flight program of the missile is much disturbed thereby. The method of natural separation, although very attractive, therefore involves operational contingencies which are difficult to accept, particularly for a modern weapon system.
In order to overcome these drawbacks, various improvements have been envisaged, consisting in introducing new forces propitious to separation, such as those generated by the opening of a braking parachute or by extraction springs provided in the sliding connection.
However, such arrangements require a logic for triggering and the above-mentioned drawbacks concerning the active method of separation are reintroduced. Moreover, it is already no longer question of natural separation.
It is an object of the present invention to overcome these drawbacks. It enables a temporary power unit to separate naturally from the rest of a missile, safely, without employing controllable auxiliary devices.